Hot answers tagged

31

Not quite, an isotope has same number of protons ($ A- N = Z = \mathrm{constant}$), but a different number of neutrons ($\mathrm N$ varies; e.g. $\ce{^3_\color{red}{1}H}$ and $\ce{^2_\color{red}{1}H}$, or $\ce{^235_\color{red}{92}U}$ and $\ce{^238_\color{red}{92}U}$ are isotopes). An isobar has a fixed number of total nucleons ($Z + N = A = \mathrm{constant}...


29

Approximately 99.3% of uranium on Earth is the $\mathrm{^{238}U}$ isotope, and this specific isotope has an atomic mass of $\mathrm{238.05\ u}$, where $\mathrm{u}$ is the atomic mass unit, equivalent to 1/12 the mass of a $\mathrm{^{12}C}$ atom. Including the other isotopes to obtain the average atomic mass drags the value down a little, but it still ends up ...


26

Harold Urey and George Murphy used spectroscopy to identify deuterium late in 1931, announcing it at the 1931 Christmas meeting of the American Physical Society. Picking up out of 'From Nuclear Transmutation to Nuclear Fission, 1932-1939" by Per F. Dahl: If anything, the naming of the new isotope proved more problematic than its isolation. At a special ...


25

Yes, $\ce{T2O}$ has been prepared and is available in significant quantity. When relatively pure, the energy released by the radioactive decay process is so intense that $\ce{T2O}$ will boil. It must be transported in a shielded, cryogenic dewar. A significant difference between compounds containing an element bonded to protium, deuterium or tritium is ...


25

In addition to the reasons ste listed, the isotopes of hydrogen have the greatest differences in mass compared to other elements. Consider that deuterium is twice as heavy as protium, and tritium is three-times as heavy as protium. Isotopes of all elements can be used in kinetic isotope experiments. The dramatic differences in mass among the hydrogen ...


24

Yes, it has a lot to do with mass. Since deuterium has a higher mass than protium, simple Bohr theory tells us that the deuterium 1s electron will have a smaller orbital radius than the 1s electron orbiting the protium nucleus (see "Note" below for more detail on this point). The smaller orbital radius for the deuterium electron translates into a shorter (...


24

For the reasons explained in New point of view on the meaning and on the values of $K_\mathrm{a}(\ce{H3O+, H2O})$ and $K_\mathrm{b}(\ce{H2O, OH-})$ pairs in water Analyst, February 1998, Vol. 123 (409–410), the $\mathrm{p}K_\mathrm{a}$ of $\ce{H3O+}$ in $\ce{H2O}$ and the $\mathrm{p}K_\mathrm{a}$ of $\ce{D3O+, D2O}$ are undefined. The entire point of the ...


24

All possible arrangements of $\ce{Br2}$ molecule: $\displaystyle 79 + 79 = 158$ $\displaystyle \color{red}{79 + 81} = 160$ $\displaystyle \color{red}{81 + 79} = 160$ $\displaystyle 81 + 81 = 162$ The amount of $\ce{^{79}Br}$ and $\ce{^{81}Br}$ in nature is roughly the same, thus each permutation is equally probable. There are two arrangements that lead to $...


21

IR-3.3.1 Isotopes of an element The isotopes of an element all bear the same name (but see Section IR-3.3.2) and are designated by mass numbers (see Section IR-3.2). For example, the atom of atomic number 8 and mass number 18 is named oxygen-18 and has the symbol $\ce{^{18}_{}O}$. IR-3.3.2 Isotopes of hydrogen Hydrogen is an exception to the rule in ...


19

Yes, there are seven known isotopes of hydrogen, though only two ($\ce{^1H}$ and $\ce{^2H}$) are stable with respect to nuclear decay, and only three ($\ce{^1H}$, $\ce{^2H}$ and $\ce{^3H}$) exist/can be made in enough quantities to be relevant outside of nuclear physics. All other hydrogen isotopes have extremely small half-lives. The next most stable ...


18

There's a very simple test for $\ce{D_{2}O}$ that springs to mind - ice cubes made with heavy water sink in light water. I assume you're talking about differentiating a glass of essentially pure heavy water from, say, tap water, in which case this test should work rather well and requires equipment no more sophisticated than a freezer, ice cube tray and some ...


17

The relative natural abundance of isotopes is not the same everywhere. Depending upon what you mean by "everywhere", there are two cases to consider. Extraterrestial Dust from before the sun was formed (stardust, presolar grains) has a very different elemental and isotopic composition than that found on earth. Depending where a star is in its life cycle ...


17

The short answer is nuclear binding energy, which is the energy needed to disassemble an atom into its subatomic parts (or in some cases the energy released when this happens). The binding energy is a consequence of the strong and weak nuclear forces that hold atoms together. Where does this energy come from? It comes from the mass of the nucleons! What? ...


17

I think there are two reasons. First, it is more convenient to categorize them under the actual element-name to which they belong. If I say "15-Beryllium" everyone knows immediately, what I'm talking about. If we add hundreds of isotope-names, it would be quite a mess. Leading to the second reason: Xenon for example has over known 30 isotopes. There are just ...


17

Your chemistry teacher is making a few simplifications there that make the statement false on a black-and-white true-and-false scale. Protons would repel each other electrostaticly due to their same charges. Neutrons interact with protons by the so-termed strong interaction (because it is stronger than the weak interaction; props to physicists for inventing ...


15

It depends how pure you want the $\ce{D2O}$ to be and what you consider simple :) Electrolysis of water strongly favors "H" being converted to hydrogen gas rather than "D", by a factor of about 8 to 1 (depending upon the electrodes). Inexpensive Equipment for the Preparation and Concentration of Pure $\ce{D2O}$ Ohio Journal of Science volume 41, number 5,...


15

Water has formula H2O. Oxygen has 3 stable isotopes (99.76% 16O, 0.039% 17O, 0.201% 18O), and hydrogen has two (99.985% 1H, 0.015% 2H). Thus, there are 9 natural isotopic configurations for water: 3 possibilities for oxygen, multiplied by 3 possibilities for 2 hydrogens with 2 possible isotopes. Out of those 9 possible configurations, only 4 have a natural ...


15

For elements with no stable isotope (i.e. Francium, Radium, and Actinium), the atomic mass is chosen to be that of the longest lived isotope. More generally, the masses for stable elements are reflective of the natural abundance of each isotope in a sample of the element. Sodium has more than one isotope, so the statement is not really true, though only ...


14

Isovalent isotopes will have the same force constant. However the different masses of the isotope will affect the position of the vibrational state in its potential well. You can rationalise the difference in well depth briefly using the vibrational frequencies of a classical oscillator as the harmonic approximation to the asymmetric well for low lying ...


14

You have two equations and 3 unknowns, so you can't solve it with just that. Say a, b, c are the fractions (as a decimal) of each isotope... $$ a(x) + b(x+1) + c(x+2) = (x+\frac{1}{2}) $$ $$a + b + c = 1 $$ The 4:1:1 solution works. Another that works is 3:0:1. Another is 7:4:1. There are infinitely many solutions.


13

When you perform 1H-NMR spectroscopy in solution, there are many cases where you want to use deuterated solvent, so that signals coming from solvent hydrogen nuclei don’t interfere with the signal from your target molecule. Hence the frequent use of deuterated water, deuterated acetone, deuterated methanol, and deuterated chloroform. Aprotic solvents are ...


13

This is an interesting question and depending on how you define bond strength the answer is different. Let us for simplicity consider only diatomic molecules and let us assume that the electronic potential between the two atoms is well described by a Morse potential \begin{align} V(r) &=D_e \left( 1 - \mathrm{e}^{-a(r-r_e)} \right)^2,& \text{with } ...


12

This is an interesting question and you raise a number of points, let's step through them. A consequence of this is that relative atomic masses of elements mined—those with two or more stable isotopes—will no longer be faithful to our current periodic table. But this is already happening. $\ce{^235U}$ constitutes 0.72% of uranium found on earth and ...


12

No, deuterium is completely stable. I found the answer at Hyperphysics, and it has to do with the mass energies of the products and reactants of this hypothetical reaction. The decay of deuterium would be $$\ce{D -> P + N + e + \bar{\nu}}_e$$ where $\ce{D}$ is deuterium, $\ce{P}$ is a proton, $\ce{N}$ is a neutron, $\ce{e}$ is an electron and $\ce{\bar{\...


12

This early paper reports that acidic compounds (phenols, carboxylic acids, and others) are noticeably more dissociated in $\ce{H2O}$ than $\ce{D2O}$. Since water (protio or deuterio) is just another example of a weak acid, it is not unreasonable to expect $\ce{H2O}$ and $\ce{D2O}$ to follow this pattern. This would suggest that the equilibrium constant in ...


12

As already stated, the system of equations is underdetermined. But we can get the range of possible solutions. Starting with \begin{equation} xM + y(M+1) + z(M+2) = M + \frac{1}{2} \end{equation} and using the normalization constraint \begin{equation} x + y + z = 1 \end{equation} we get \begin{equation} (x+y+z)M + y + 2z = M + \frac{1}{2} \\ y + 2z = \...


12

Periodic tables of elements (PTEs) are often abused by designers. Books are more trustworthy as long as they are written by scientists. Long story short, the second notation $(\ce{^{12}_{6}C})$ is the correct one. There is an easy to remember AZE notation: $^A_Z\ce{E}$. I suspect the PTE you were looking at lists standard (averaged) atomic weights of the ...


11

The differences in molecular mass stem from two sources: Nuclear binding energy The definition of the atomic mass unit and Avogadro's number (and thus the g/mol), is the mass of one atom of the carbon-12 isotope $\ce{^{12}_6 C}$. 1 amu = $\frac{1}{12}$ the mass of one atom of $\ce{^{12}_6 C}$ and the g/mol unit is based on the definition of Avogadro's ...


11

ORCA cannot read isotope information from XYZ files -- the OpenBabel specification does not include it and so ORCA doesn't look for it in the geometry file. You need to copy-paste the geometry into your input file: * xyz 0 1 ... H 0.55630745024474 -2.41808278363585 -1.72738492075924 M 2 * Alternatively, if you are only interested in the ...


11

According to the IUPAC organic chemistry nomenclature 2013, section P-83 ISOTOPICALLY LABELED COMPOUNDS, it has to do with uniformly labeled compounds: P-83.5 GENERAL AND UNIFORM LABELING P-83.5.1 In the name of a selectively labeled compound in which all positions of the designated element are labeled, but not necessarily in the same isotopic ...


Only top voted, non community-wiki answers of a minimum length are eligible